This invention relates generally to optical disc drives used for data storage, and more specifically to signals used for determining radial position of an optical head.
In optical disc drives, for example drives for Compact Discs (CD) and Digital Versatile Disc (DVD), one or more light beams (typically from a laser diode), illuminate one or more spots on the disc, and are reflected back into an optical head. For optical media with lands and grooves, the reflectivity of the disc surface changes when the laser illuminates a land, versus a groove. Similarly, for optical media with pitted data, the reflectivity of the disc surface changes when the laser illuminates the data tracks, versus space between data tracks.
Typically, an optical detector assembly in the optical head provides multiple signals. It is common for an optical detector to have multiple segments, and for multiple signals to be generated as sums or differences of signals from individual segments. It is also common to have multiple detectors, each of which has multiple segments, within one assembly.
In general, the following signals are required: a data signal, a focus error signal, and a tracking error signal. The tracking error signal indicates whether the laser spot is centered on a data track. In general, a drive must also be able to detect track crossings as the optical head sweeps radially across tracks. One inexpensive approach is to detect peaks or zero-crossings of the tracking error signal as the optical head sweeps radially. If errors occur, a re-seek may be required. To reduce errors, it is desirable to have a signal that indicates direction of radial movement.
It is common in control systems to generate two periodic signals, each having the same period, that are out of phase by one-fourth of a period (called quadrature), and to combine information from the two signals to provide both position information and direction of movement. It is common in optical disc drives to generate two quadrature signals as a function of radial movement. Typically, the tracking error signal is approximately a sinusoidal waveform, which has a magnitude of zero when the spot is centered on a data track, and increases when off-track in one direction and decreases when off-track in the other direction. As one alternative for generating a separate radial tracking signal for quadrature, one can differentiate the tracking error signal to provide a signal that is out of phase by one-fourth period. However, differentiation is frequency dependent, sensitive to noise, and does not work well with slow radial movement. Another alternative for generating a separate radial tracking signal for quadrature is to use a low-pass filtered version of the data signal (also known as a tracking contrast signal). For some media, when the optical head is swept radially across tracks, the tracking contrast signal is also approximately a sinusoid, with a peak magnitude when centered over data, and decreasing when off-track from the center of data. If the center of a data track is the reference position for radial movement, then the tracking error signal is approximately a sine wave, and the tracking contrast signal is approximately a cosine signal. Accordingly, the tracking error signal and the tracking contrast signal can be used as two quadrature signals for determining radial position and direction of movement. However, for phase-change rewriteable media, if no data has been written, a tracking contrast signal may not be suitable.
Within the optical head, a radial position adjuster may provide fine movement of a lens to radially adjust the position of the focused laser spot. When the objective lens is displaced from a nominal center position, it may result in a DC offset in the tracking error signal (one common term for the phenomena is xe2x80x9cbeamwalkxe2x80x9d). Radial tilt of the disc may also cause a DC offset in the tracking error signal. There may also be other contributors to a DC offset in the tracking error signal.
One known solution to the problem of DC offset in the tracking error signal is 3-spot tracking (also called differential push-pull, or DPP), in which two satellite illumination spots are positioned so that each of the satellite spots provide a signal that is 180 degrees out of phase with a main spot. The satellite spots are also subject to DC offset. Signals from the satellite spots are added together, the sum is multiplied by a suitable multiplier, and the result is subtracted from the signal from the main spot. As a result, the DC offsets from the signals from the satellite spots are subtracted from the DC offset of the signal from the main spot. In the following example equations, it is assumed that the signal magnitude from the main spot is ten times the signal magnitude from the satellite spots.
Main spot signal=MS=sin(xcfx89x)+Offset
(where x is a radial distance and xcfx89 is a function of track pitch)
Right satellite spot signal=RS=0.1[xe2x88x92sin(xcfx89x)+Offset]
Left satellite spot signal=LS=0.1[xe2x88x92sin(xcfx89x)+Offset]
MSxe2x88x925(RS+LS)=
sin(xcfx89x)+Offsetxe2x88x925{0.1[xe2x88x92sin(xcfx89x)+Offset]+0.1[xe2x88x92sin(xcfx89x)+Offset]}=
2 sin(xcfx89x)
From the above, DPP can provide a tracking error signal with no DC offset.
It is also known to provide two satellite spots positioned to provide a signal that is out of phase with a track crossing signal by one-fourth of the period. See, for example, U.S. Pat. No. 5,159,589 (Ohsato). In Ohsato, the main spot provides a sine signal, the satellite spots provide cosine signals, and the sine signal is divided by a sum of the cosine signals, resulting in a tangent function. The quotient of the two signals eliminates amplitude sensitivity because amplitude changes affect both the sine and cosine signals by the same proportion.
There is a general need, for elimination of DC offset in a tracking error signal, and for generating a signal that provides direction of movement for radial track crossing. There is a particular need for elimination of DC offset and generation of quadrature signals for phase-change rewriteable media, which may not have any data written on the disc.
In drives in accordance with the invention, signals from two satellite spots are used to cancel DC offset from a tracking error signal, and for generating a quadrature signal for determining direction of radial movement. In contrast to DPP, the two satellite spots generate signals that are out of phase with the main tracking error signal by approximately one-fourth period, as in Ohsato. In contrast to Ohsato, the signals from the satellite spots are used to provide DC offset information and a quadrature signal.